PCI/DPC: Rename interrupt_event_handler() to dpc_work()

Rename interrupt_event_handler() to dpc_work() so there's more useful
information in stack traces and similar situations.  No functional change
intended.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Sinan Kaya <okaya@codeaurora.org>